Natural gas liquids recovery process

ABSTRACT

Methods and systems for operating and NGL recovery process are provided. In an exemplary method, an absorber column upstream of a fractionator column is operated at a higher pressure than a pressure in the fractionator column. An NGL (C3 plus) stream is taken from the bottom of a fractionator column and then ethylene/ethane stream is taken from the top of the fractionator column. A differential pressure between the absorber column and the fraction are column is controlled based, at least in part, on a flow rate of the fractionator feed stream from the absorber column to the fractionator column.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/590,806, filed on Oct. 2, 2019, which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

The present disclosure relates to gas plants that are used for theproduction of natural gas liquids.

BACKGROUND

The processing of natural gas to prepare for pipeline sales orliquefaction involves a number of separations to isolate methane fromother components or contaminants, such as crude oil, liquid water, watervapor, carbon dioxide, hydrogen sulfide, and other hydrocarbons. Theseparation of liquid water may be performed by a settling tank. Theinitial separation from crude oil, condensates, or both, if needed, maybe performed by flashing or distillation. Other contaminants, includingmercury and chlorides, among others, may be removed by techniques knownin the art, such as wash columns.

Removal of acid gases, such as carbon dioxide, hydrogen sulfide, andothers, is termed sweetening. Sweetening may be performed by passing thenatural gas through an adsorption column, such as an amine absorber, toremove the acid gases from the natural gas. Newer technologies forsweetening have also been implemented, including gas membraneseparators, among others.

Once the contaminants have been removed, methane, ethane, and ethylenemay be separated from the other low boiling point hydrocarbons. Thesemay include propane, butanes, and small amounts of higher molecularweight hydrocarbons, such as C₅ and C₅ plus. These materials arecollectively termed natural gas liquids or NGL. The separation isgenerally performed by fractionation, often including low temperaturedistillation processes.

SUMMARY

An embodiment described in examples herein provides a method foroperating a natural gas liquids (NGL) plant. The method includesoperating an absorber column upstream of a fractionator column at ahigher pressure than a pressure in the fractionator column. An NGL (C3+)stream is taken from a bottom of a fractionator column, and anethane/ethylene stream is taken from a top of the fractionator column. Adifferential pressure between the absorber column and the fractionatorcolumn is controlled based, at least in part, on a flow rate of afractionator feed stream from the absorber column to the fractionatorcolumn.

In an aspect, the pressure differential between the absorber column andthe fractionator column can be controlled to keep the pressure in theabsorber column to less than about 10% above the pressure in thefractionator column. Controlling the differential pressure may includeadjusting the pressure on the fractionator column, while holding thepressure on the absorber column steady. Controlling the differentialpressure may include adjusting the pressure on the absorber column whileholding the pressure on the fractionator column steady.

In an aspect the method includes increasing the bottom temperature ofthe absorber column to increase flashing of C1 and C2. A reflux flow maybe increased in the absorber column. A reflux flow may be increased inthe fractionator column. A temperature of a feed gas to the absorbercolumn may be increased to increase the stripping rate. A bottomtemperature of the fractionator column may be decreased to decreasestripping of C3 plus compounds.

In an aspect the method includes increasing the differential pressure toincrease the flow rate of the fractionator feed stream. The differentialpressure may be decreased to decrease the flow rate of the fractionatorfeed stream.

In an aspect, the method includes measuring the flow rate of thefractionator feed stream, an overhead pressure in the absorber column,and an overhead pressure in the fractionator column. A new differentialpressure is calculated based, at least in part, on a new set point forthe flow rate of the fractionator feed stream. A first set point iscalculated for an absorber column pressure controller and a second setpoint is calculated for a fractionator column pressure controller. Thefirst set point is sent to the absorber column pressure controller andthe second set point is sent to the fractionator column pressurecontroller.

In an aspect, the method includes setting an absorber column pressurecontroller to about 390 psig and setting a fractionator column pressurecontroller to about 350 psig.

In an aspect, the method includes deactivating a feed pump upstream ofthe fractionator column. The feed pump upstream of the fractionatorcolumn may be removed.

In an aspect, the method includes maintaining a fractionator columnpressure controller at about 350 psig, and adjusting an absorber columnpressure controller to adjust the flow rate of the fractionator feedstream to the fractionator column. A quench flow to the absorber columnmay be adjusted to enhance separation in a rectifying section.

Another embodiment described in examples herein provides a controlsystem for operating columns in a natural gas liquids (NGL) plant. Thecontrol system includes a flow sensor to measure a flow rate of afractionator column feed stream and an absorber column pressurecontroller. The absorber column pressure controller includes an absorbercolumn pressure sensor and an absorber column pressure control valve.The control system also includes fractionator column pressure controllerthat includes a fractionator column pressure sensor, and a fractionatorcolumn pressure control valve. A controller is included in the controlsystem. The controller includes a sensor interface to obtainmeasurements from the flow sensor, the absorber column pressure sensor,and the fractionator column pressure sensor. The controller alsocontains a controller interface to communicate set points to theabsorber column pressure controller and the fractionator column pressurecontroller. A processor is included in the controller to execute storedinstructions. A data store in the controller includes instructionsconfigured to direct the processor to read measurements from the flowsensor, the absorber column pressure sensor, and the fractionator columnpressure sensor and calculate a set point for the absorber columnpressure controller based, at least in part, on a set point for the flowrate. Instructions are included in the data store to instruct theprocessor to adjust the set point for the absorber column pressurecontroller to match the calculated value.

In an aspect, the data store includes instructions configured to directthe processor to calculate a pressure differential between an absorbercolumn and a fractionation column based, at least in part on the setpoint for the flow rate. Instructions are also included to calculate anew value for the set point for the absorber column pressure controllerbased, at least in part, on the pressure differential and adjust the setpoint based on the new value for the set point for the absorber columnpressure controller.

In an aspect, the data store includes instructions configured to directthe processor to maintain the set point for the fractionator columnpressure controller, while providing the new set point to the absorbercolumn pressure controller.

In an aspect, the data store includes instructions configured to directthe processor to calculate a set point for the fractionator columnpressure controller based, at least in part, on the set point for theflow rate, and adjust the set point for the fraction or column pressurecontroller to match the calculated set point for the fractionator columnpressure controller.

In an aspect, the data store includes instructions configured to directthe processor to calculate a set point for the fractionator columnpressure controller, based at least in part, on a set point for the flowrate and adjust the set point for the fractionator, pressure control ofthe match the calculated set point for the fractionator column pressurecontroller.

Another embodiment described in examples herein provides a natural gasliquids (NGL) plant. The NGL plant includes a chiller upstream of agas-liquid separator, a gas line fluidically coupling a gas flow fromthe gas-liquid separator to an absorber column through a gas side of aheat exchanger, and a liquid line fluidically coupling the liquid flowfrom the gas-liquid separator to a fractionator column through a pumpand the heat exchanger, where the liquid line couples to an oppositeside of the heat exchanger from the gas line. A bottoms line from theabsorber column fluidically couples to the liquid line to thefractionator column. A flow sensor on the liquid line, downstream of theheat exchanger, measures a flow rate of the fractionator column. Anabsorber column pressure sensor is located on an overhead line from theabsorber column. An absorber column pressure control valve is located onthe overhead line from the absorber column. A fractionator columnpressure sensor is located on an overhead line from a fractionatorreflux drum. A fractionator column pressure control valve is located onthe overhead line from the fractionator accumulator.

In an aspect, the NGL plant includes a controller. The controllerincludes a sensor interface to obtain measurements from the flow sensor,the absorber column pressure sensor, and the fractionator columnpressure sensor. The controller includes a controller interface tocommunicate set points to the absorber column pressure controller in thefractionator column pressure controller. The controller includes aprocessor configured to execute stored instructions and a data storeincluding instructions configured to direct the processor to readmeasurements from the flow sensor, the absorber column pressure sensor,and the fractionator column pressure sensor. The data store includesinstructions to direct the processor to calculate a set point for theabsorber column pressure controller based, at least in part, on a setpoint for the flow rate, and adjust the set point for the absorbercolumn pressure controller to match the calculated value for the setpoint for the absorber column pressure controller.

In an aspect, the NGL plant includes a liquid product pump coupled to abottom of the fractionator column wherein the liquid product pumpexports an LNG product.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are a simplified process flow diagram of an NGL plant.

FIG. 2 is a flow chart of a method for operating the NGL plant.

FIG. 3 is a block diagram of a controller for controlling the columnpressures in the NGL plant.

FIG. 4 is a plot of pressure versus time showing a change in theoperating characteristics for the columns in the NGL plant.

FIG. 5 is a plot of flow rate for reflux to an absorber column versustime showing the change in operating characteristics for the NGL plant.

FIG. 6 is a plot of compositions of products for the NGL plant in molepercent versus time, showing the change in compositions during theprocess change.

DETAILED DESCRIPTION

In a natural gas liquids (NGL) recovery process, propane and highercarbon compounds (C₃ plus) are extracted from a feed gas that alsoincludes methane and ethane. To perform the process, the feed gasundergoes a series of cooling steps prior to being processed in anabsorber column and fractionator column that performed the separation.The methane and ethane, generally in a single outlet stream, are thensent to a C₂ recovery plant, and the C₃ plus compounds are either soldor processed for further extraction, such as the isolation of C₃compounds, C₄ compounds, C₅ compounds, and the like.

The feed gas may be sweetened by the removal of acid gases prior tocooling for the fractionation process. In some examples, the feed gas iscompressed prior to beginning of process to a limit above columnpressure and to accommodate the pressure drop across the plant, such asif the feed pressure is below about 440 kPa (about 50 psig), below about790 kPa (about 100 psig), below about 1480 kPa (about 200 psig), orbelow about 2860 kPa (400 psig), or higher. The number of cooling stagevaries, based on the target temperature for fractionating the feed gas,such as about −100° C. (about −148° F.), about −30° C. (about 5° F.)about −15° C. (about 5° F.), or about 0° C. (about 32° F.). Typically,the cooling is performed in three stages. Dehydrators are installedbefore the cryogenic process chilling stages, for example, after thefirst stage chilling is finished, to reduce water content to a low ppmlevel to avoid hydrate formation, such as about 50 ppm, about 25 ppm,about 10 ppm, about 5 ppm, about 1 ppm, or lower. In addition, a numberof heat exchangers are installed to cool the feed gas as part of theheat integration between the colder streams farther downstream in theprocess and the harder streams farther upstream in the process.

After the feed gas is dehydrated and chilled to the target temperature,it is then fed into the absorber column. Typically, the absorber columnis upstream of the fractionator column, and is intended to directlyknock out unstable condensate and flash off light ends without havingboil off liquid in the bottom. As used herein, “unstable condensate”refers to C₃ plus compounds, while “light ends” refers to C₂ and C₁compounds, such as ethane, and methane.

Further separating of the light ends compounds from the bottom steam ofC₃ plus is the purpose of the fractionator column. The fractionator hastwo reboilers at the bottom and a reflux system, including anaccumulator, in the overhead. The accumulator is also termed a refluxdrum, herein. A portion of the reflux stream from the accumulator is fedto the absorber to decrease the heavy end, such as C₃ plus compounds,from being carried over in the overhead stream. The unstable condensatefrom absorber, and from upstream separators, is fed to the fractionatorcolumn using reflux pumps. In the fractionator, the feed is furtherstabilized to produce on spec C₃ plus compounds from the bottom andflashing of the C₁ and C₂ compounds from the top. As used herein,“stabilized” refers to the removal of heavier compounds that mayseparate out in further operations on the C₁ and C₂ compounds.Generally, the fractionator is operated at a higher pressure than theabsorber. Accordingly, a pump is used to deliver condensate fromabsorber column and other sources which are operated at lower pressure.

In examples described herein, the operating pressure of the absorbercolumn is higher than fractionator column. This decreases or eliminatesthe need for pumps to provide feed to the fractionator column. At thetested boundary conditions, described in examples herein, the productspecification is maintained or improved by adjusting other processvariables in addition to the column pressure such as reflux rate, bottomtemperature, and the differential pressure between two columns. Thepressure of the product streams, including the overhead and bottomproduct streams, are sufficient to provide material to downstreamprocess, such as at about 300 psig, 350 psig, 400 psig, or higher.

FIG. 1 is a simplified process flow diagram of an NGL plant 100. The NGLplant 100 receives a feed stream 102 that includes a saturated sweetgas. As used herein, “saturated” indicates that the gas is saturatedwith water vapor. As used herein, “sweet” indicates that the gas hasbeen treated to remove acid gases, such as hydrogen sulfide and carbondioxide.

The feed stream 102 is compressed by a feed gas compressor 104 suctionscrubber. An aftercooler 106 cools the compressed feed gas to atemperature of less than about 50° C. (122° F.), less than about 60° C.(140° F.), or less than about 70° C. (158° F.), using ambient air as theheat exchange fluid. In an embodiment, the aftercooler 106 cools thecompressed feed gas to a temperature of about 130° F. (about 54° C.).

The compressed feed gas then enters a first stage 108 of chilling. Inthe first stage 108 of chilling, the compressed feed gas is cooled downto less than about 20° C., less than about 25° C., or less than about30° C., or higher in heat exchangers and a chiller which are operated inseries. In the first heat exchanger 110, the compressed feed gas iscooled against the residue gas, carried in a residue gas line 112. Thecompressed feed gas is cooled in the first chiller 113 against propanerefrigerant, before being sent to a three phase separator 114. In thethree phase separator 114, hydrocarbon condensate and free water areseparated from gaseous hydrocarbons. The hydrocarbon condensate isrecycled to condensate stripper through a condensate line 116 and thefree water is sent through a water line 118 for purification anddisposal. The compressed feed gas is then fed to another chiller 120 forfurther cooling down prior to entering a second three phase separator122 to remove further amounts of water and hydrocarbon.

From the second three phase separator 122, the compressed gas feed isfed to a dehydrator 124, in which molecular sieves, or other adsorbents,remove moisture from hydrocarbon vapors. After the dehydrator 124, thecompressed gas feed includes less than about 10 ppm water, less thanabout 5 ppm water, less than about 1 ppm water, or lower. This helps toprevent hydrate formation at the lower processing temperatures furtherdownstream. In an embodiment, compressed gas feed, termed dry gas,leaves the dehydrators at about 18° C. (65° F.) in a dry gas line 126.

Depending on the content of higher carbon number compounds, such as C₂plus, in the dry gas, a bypass line 128 may be used to send the dry gasdirectly to a booster compressor 130. From the booster compressor 130,the dry gas may be sent to a C₂ recovery plant through a C₂ feed line132. In some embodiments, the bypass line 128 is used if the dry gasincludes less than about 1 vol. %, less than about 0.5 vol. %, less thanabout 0.1 vol. %, or lower. Further, the bypass line 128 may be used ifthe downstream separation equipment is being serviced.

To further chiller stages, a second stage 134 and a third stage 136 areused for cooling the dry gas to less than about −20° C., less than about−25° C., less than about −30° C., or less than about −35° C., before thedry gas is introduced to an absorber column 138. In an embodiment, thedry gas is cooled to about −32° C. (−25° F.) before it is introduced tothe absorber column 138 as a cold feet gas.

The second stage 134 consists of two heat exchangers 140, where thesource of cooling is from residue gas, and a chiller 142, where sourceof cooling is from propane refrigerant. The second stage 134 cools thefeed gas to less than about −5° C., less than about −10° C., less thanabout −15° C., or less than about −20° C. In an embodiment, the dry gasis cooled to about −11° C. (about 12° F.), before it is introduced to agas-liquid separator 144. Similarly, the third stage 136 includes twoheat exchangers 146 and a chiller 148. The source of cooling for theheat exchangers 146 is from residue gas and unstable condensate from theabsorber column 138. The source of cooling for the chiller 148 is frompropane refrigerant.

The cold feed gas from the chiller 148 is introduced to the absorbercolumn 138 through a gas feed line 150. Further, a liquid heavy end froma fractionator reflux drum 152 is fed to the absorber column 138 througha liquid line 154. An overhead line 156 from the absorber column 138removes an absorber overhead stream that mainly includes methane andethane.

The condensate carried by the bottoms line 158 from absorber column 138is pumped by the absorber bottoms pump 160 and combined with the coldcondensate coming from the gas-liquid separator 144 through a separatorpump 162. The total condensate flow is preheated in second of the heatexchangers 146, before being fed to the fractionator column 164 forfurther stabilization through a fractionator feedline 166. As usedherein, “stabilization” refers to the removal of higher carbon numbercompounds, such as C₃ plus, which may condense due to cooling frompressure decreases in downstream processes.

The fractionator column 164 has rectifying section 168 on the top andstripping section 170 on the bottom. In the stripping section 170,re-boiling takes place in two reboilers 172 which are heated by steam.The NGL product is removed from the column bottom through a NGL productline 174 and fed to downstream processes through one or more NGLshipping pumps 176. The overhead vapor from the fractionator column 164,exits through an overhead line 178, and is partially condensed infractionator condenser 180. The fractionator condenser 180 is cooled bypropane to a temperature of about −30° C., to about −40, or to about−50° C. In an embodiment, the temperature of the fractionator condenser180 is set to about −39° C. (about −39° F.).

The fractionator condensate is collected in a collected in thefractionator reflux drum 152. Liquid condensate from the fractionatorreflux drum 152 provides reflux to the absorber column 138 and thefractionator column 164 through reflux pumps 182. Off gas fromfractionator reflux drum 152 contains mostly ethane and methane. The offgas exits the fractionator reflux drum 152 through an overhead line 184and is combined with overhead vapors in the overhead line 156 from theabsorber column 138 to form the plant residue gas stream in the residuegas line 112. As described herein, the plant residue gas stream in theresidue gas line 112 is used to precool feed gas back to upstreamthrough a series of heat exchanger, prior to being fed to the boostercompressor 130.

The pressure in the absorber column 138 is controlled by an absorbercolumn pressure control valve (PCV) 186 located on the overhead line 156from the absorber column 138. The absorber column PCV 186 is controlledby an absorber column pressure controller 188 that includes an absorbercolumn pressure sensor on the overhead line 156 to sense the pressure.The pressure in the fractionator column 164 is controlled byfractionator column pressure control valve (PCV) 190 located on theoverhead line 184 from the fractionator reflux drum 152. Thefractionator column PCV 190 is controlled by a fractionator columnpressure controller 192 that includes a fractionator column pressuresensor on the overhead line 184 from the fractionator reflux drum 152.

Generally, the NGL plant 100 maintains the pressure of the absorbercolumn 138 at about 2620 kPa (about 365 psig) and the fractionatorcolumn 164 at about 2860 kPa (about 400 psig). Accordingly, the absorberbottoms pump 160 and the separator pump 162, collectively referred to asthe fractionator feed pumps 194, are used to provide feed from the lowerpressure of the absorber column 138 to the higher pressure of thefractionator column 164. As a result, the delivery pressure for the gasproduct to a C₂ plant or the liquid LNG product to an LNG separationfacility, remain in a favorable range.

In embodiments described herein, the column pressure in the absorbercolumn 138 is set to be higher than the pressure in the fractionatorcolumn 164. This is described further with respect to FIG. 2. Acontroller 196 that is coupled to the pressure controllers 188 and 192is used to control the pressures to maintain the feed flow from theabsorber column 138 to the fractionator column 164, and, thus, theliquid outlet rate. Currently, the two pressure controllers 188 and 192work independently, although both are automatically controlling. Invarious embodiments, the controller 196, or other calculator block, isused to keep the differential pressure to a level that is justsufficient to provide flow from the absorber column 138 to thefractionator column 164.

A flow sensor for the fractionating column feed stream, the fractionatorcolumn feed flow sensor 198, is located on the fractionator feedline 166to provide the flow rate of the feed to the fractionator column 164 tothe controller 196. The controller 196 is discussed further with respectto FIG. 3. Examples of the operation of the new set points are discussedwith respect to FIGS. 4, 5, and 6. As described herein, this mode ofoperation may decrease the use of the fractionator feed pumps 194, andmay allow their removal from the NGL plant 100.

FIG. 2 is a flow chart of a method 200 for operating the NGL plant 100.The method 200 begins at block 202 when the pressure is raised on theabsorber column. The pressure may be set to about 2785 kPa (about 390psig).

At block 204, the pressure is lowered on the fractionator column toabout 2520 kPa (about 350 psig). The pressure differential between theabsorber column and the fractionator column allows feed to flow from theabsorber column, and the upstream separator, to the fractionator columnwithout any required pumping. Accordingly, the fractionator feed pumps194, described with respect to FIG. 1, may be eliminated. The refluxrate to the fractionator column is increased to compensate for a higherstripping rate in fractionator due to lower overhead pressure.

The new settings as optimum condition is about 2785 kPa (about 390 psig)for absorber and about 2520 kPa (about 350 psig) for fractionator todeliver flow from absorber and upstream separator to fractionatorwithout any required pump. The differential pressure is adjustable basedon liquid production. Moreover the feed gas compressor dischargepressure would be kept within the operation envelope while fractionatorpressure at about 2520 kPa (about 350 psig) is adequate to dispatch theNGL product without a shipping pump, since the NGL header is much lowerthan this limit.

At block 206, the bottom temperature of the absorber column is increasedby increasing the absorber feed temperature from −32° C. to −26° C. Thisto increase the flashing off of C₁ and C₂ compounds in the absorbercolumns. The rate of stripping of C₃ plus compounds is controlled byincreasing reflux flow from overhead. In embodiments, the reflux, orquench, rate is increased to compensate for the increase in stripping.By doing so, the cooling required to precool the feed stream prior toentering absorber is reduced, and the temperature feed to absorber fromsecond stage chiller is also increased to increase stripping rate. Theoverhead pressure tends to be increased by the increase in the strippingof the light end (C₁ and C₂). The increase of the reflux rate doesincrease the cooling duty on the reflux system, but lower than coolingduty on the feed absorber chillers.

At block 208, the bottom temperature of the fractionator column isdecreased from 66° C. to 60° C. This decreases the rate of stripping ofpropane from the bottoms of the fractionator column while maintainingadequate stripping rate of C₁ and C₂ to maintain the NGL withindesirable specifications. The amount of steam consumption used in thereboiler is decreased as a result. The reflux into the fractionatorcolumn is also increased to decrease the stripping.

At block 210, the differential pressure between the absorber column ofthe fractionator column is controlled based, at least in part, on theflow rate from the absorber to the fractionator column feed stream. Thedifferential pressure is varying from 40 psig to 25 psig between the twocolumns based on the plant throughput. In some embodiments, thedifferential pressure may also be controlled, based at least in part, onthe NGL outlet flow rate, for example, the loading on the plant. Anadvanced controller is used to set the pressure differential to be justsufficient to drive liquid from the absorber column to the fractionatorcolumn. If the plant loading is low, then the required differentialpressure between the columns is also low. As it is not practical tooperate the columns at floating pressure, the controller can anticipatedisturbances from changing plant loading and composition.

The differential pressure may be adjusted based on liquid production.The overhead products from both columns can be merged despite havingdifferent overhead pressure by just simply operating the pressurecontrol valves at the correct set point. With controlling differentialpressure of both column as low as just sufficient top drive liquiddelivered from absorber to fractionator, the combined pressure fromcolumn overhead remain high which favorable to the next processes. Forinstance, high suction pressure to booster compressor will help inreducing compression power.

From an energy perspective, the increase of the reflux to both columnsimplies an increase in the amount of cooling duty to the refluxcondenser. However, this is offset by lower cooling duty on the feedabsorber chiller and a decrease in steam consumption by the reboilers.Overall, energy consumption is reduced by powering down or removing thefractionator feed pumps that provide liquid from the absorber column tothe fractionator column.

FIG. 3 is a block diagram of a controller 196 for controlling the columnpressures in the NGL plant 100. Like numbered items are as describedwith respect to FIG. 1. The controller 196 may be used to provide morerobust process control and higher efficiency.

In some embodiments, the controller 196 may be a separate unit mountedin the field or plant, such as a programmable logic controller (PLC),for example, as part of a supervisory control and data acquisition(SCADA) or Fieldbus network. In other embodiments, the controller 196may interface to a distributed control system (DCS) installed in acentral control center. In still other embodiments, the controller 196may be a virtual controller running on a processor in a DCS, on avirtual processor in a cloud server, or using other real or virtualprocessors.

The controller 196 includes a processor 302. The processor 302 may be amicroprocessor, a multi-core processor, a multithreaded processor, anultra-low-voltage processor, an embedded processor, or a virtualprocessor. The processor 302 may be part of a system-on-a-chip (SoC) inwhich the processor 302 and other components are formed into a singleintegrated package. In various embodiments, the processor may includeprocessors from Intel® Corporation of Santa Clara, Calif., from AdvancedMicro Devices, Inc. (AMD) of Sunnyvale, Calif., or from ARM holdings,LTD., of Cambridge England. Any number of other processors from othersuppliers may also be used.

The processor 302 may communicate with other components of thecontroller 196 over a bus 304. The bus 304 may include any number oftechnologies, such as industry standard architecture (ISA), extended ISA(EISA), peripheral component interconnect (PCI), peripheral componentinterconnect extended (PCIx), PCI express (PCIe), or any number of othertechnologies. The bus 304 may be a proprietary bus, for example, used inan SoC based system. Other bus technologies may be used, in addition to,or instead of, the technologies above. For example, plant interfacesystems may include I2C buses, serial peripheral interface (SPI) buses,Fieldbus, and the like.

The bus 304 may couple the processor 302 to a memory 306. In someembodiments, such as in PLCs and other process control units, the memory306 is integrated with a data store 308 used for long-term storage ofprograms and data. The memory 306 include any number of volatile andnonvolatile memory devices, such as volatile random-access memory (RAM),static random-access memory (SRAM), flash memory, and the like. Insmaller devices, such as PLCs, the memory 306 may include registersassociated with the processor itself. The data store 308 is used for thepersistent storage of information, such as data, applications, operatingsystems, and so forth. The data store 308 may be a nonvolatile RAM, asolid-state disk drive, or a flash drive, among others. In someembodiments, the data store 308 will include a hard disk drive, such asa micro hard disk drive, a regular hard disk drive, or an array of harddisk drives, for example, associated with a DCS or a cloud server.

The bus 304 couples the controller 196 to a controller interface 310.The controller interface 310 may be an interface to a plant bus, such asa Fieldbus, an I2C bus, an SPI bus, and the like. The controllerinterface 310 couples the controller 196 to the absorber column pressurecontroller 188 for the absorber column and the fractionator columnpressure controller 192 for the fractionating column. This allows thecontroller 196 to obtain values for the pressure measurements from thepressure controllers 188 and 192, and to communicate set points to thepressure controllers 188 and 192 for controlling the pressure controlvalves 186 and 190.

In some embodiments, sensors are used in place of the pressurecontrollers 188 and 192, and the pressure control valves 186 and 190 aredirectly controlled by the controller 196 or a plant control system,such as a DCS. In these embodiments, the pressure controllers 188 and192 are virtual control blocks, or instructions, programmed in the DCS.

A sensor interface 312 couples the controller 196 to the fractionatorcolumn feed flow sensor 198 that measures the feed flown to thefractionating column. The sensor interface 312 may be integrated withthe controller interface 310 as a single serial bus connection. Othersensors may be integrated with the system for determining parametersthat may be used for the control algorithms, for example, a flow sensoron the NGL output from the bottoms of the fractionating column and aflow sensor on the residue gas.

If the controller 196 is located in the field, a local human machineinterface (HMI) 314 may be used to input control parameters. The localHMI 314 may be coupled to a display 316, such as a multiline LCDdisplay, or a display screen, among others. A keypad 318 may be coupledto the local HMI 314 for the entry of control parameters, such as thepressure differential between the absorber column pressure controller188 and the fractionator column pressure controller 192.

In some embodiments, the controller 196 is linked to a plant controlsystem, such as a DCS, through a network interface controller (NIC) 320.The NIC 320 can be an Ethernet interface, a wireless network interface,or a plant bus interface, such as Fieldbus. The controller 196 may beintegrated with the controller interface 310, wherein the controller 196is another note on the control bus coupled to the controller interface310.

The data store 308 includes blocks of stored instructions that, whenexecuted, direct the processor 302 to implement the functions of thecontroller 196. The data store 308 includes a block 322 of instructionsto direct the processor to measure the pressure in the absorber overheadusing the communications with the absorber column pressure controller188. The data store 308 also includes a block 324 of instructions todirect the processor to measure the pressure in the fractionator refluxdrum overhead from the fractionator column pressure controller 192. Thepressure in the fractionator reflux drum corresponds to the pressure inthe fractionator column. A block 326 of instructions directs theprocessor to measure the feed rate to the fractionating column using thefractionator column feed flow sensor 198 for the fractionator columnfeed.

The data store 308 includes a block 328 of instructions to direct theprocessor to calculate set points for the pressure controllers 188 and192 based on the current pressure and flow measurements, and a set pointfor the feed flow rate. This may be calculated, for example, from theplant loading measured by flow sensors on the NGL and residue gasoutlets. The data store includes a block 330 of instructions to directthe processor to adjust the set points for the pressure controllers 188and 192 to match the calculated values, for example, through thecontroller interface 310.

A block 332 of instructions may be included in the data store 308 todirect the processor to monitor the feed flow rate and adjust thepressure differential between the absorber column pressure controller188 and the fractionator column pressure controller 192 to maintain afeed flow as determined by the fractionator column feed flow sensor 198.

Any number of other blocks may be included in the data store 308 toimplement other functions, including blocks of instructions to directthe processor to measure the NGL and residue gas flow rates. Complexcontrol algorithms may also be included in blocks, such as block 332, inthe data store 308.

In some embodiments, the complex control algorithms include models ofthe effects of the pressure changes in the absorber column and thefractionator column on feed flow to the fractionating column, NGL outletflow, or residue gas flow, or any combinations thereof. The models maybe statistical models and may include static or dynamic models, orinclude elements based on correlation between the columns differentialpressure and flow from absorber to the fractionator. The models may beused to predict and adjust the set points for the absorber columnpressure controller 188 and the fractionator column pressure controller192 to maintain an appropriate differential pressure for thefractionating feed flow, the product flow, product pressure, and thelike. To maintain smooth column operation, the pressure set point changeis locked in one column and vary with second column with slow responsestep.

In an embodiment, a calculator block in the data store 308 includesinstructions to direct the processor 302 to solve a linear equation ofdifferential pressure as function of flow. Once the processor 302obtains flow input from the fractionator column feed flow sensor 198, itwill use the instructions of the calculator block to calculate therequired differential pressure for delivering a desired flow of liquidfrom the absorber column to the fractionator column. The processor 302will then use the instructions in block 332 with the resultingdifferential pressure to calculate the required differential pressurebetween the two columns. In some embodiments, one column, such as theabsorber column, will be set at a fixed pressure and used as an input toblock 332. Based on the calculated pressure differential, the output ofblock 332 is used as the set point for the pressure controller for acolumn, such as the fractionator column, which is operated at a floatingpressure.

In some embodiments, the changes to the column pressures will beperformed slowly to decrease the probability of process upsets that mayresult in decreases in product quality, for example, to maintain theproduct in an on-spec condition during the adjustments. As describedherein, the process adjustments include the changes in flux flow, inlettemperature, and bottom temperature.

For example, if the feed to the absorber column is reduced, the requireddifferential pressure is reduced to gain higher delivery pressure forthe next processes, such as the booster compressor. If one column, suchas the absorber column in this example, is set at a fixed pressure, theother column, such as the fractionator column in this example, will beoperated at higher pressure based on the results of the calculatorblock. Thus, the pressure of the fractionator column will be increasingas the set point from block 332 is incrementally increased towards thenew set point. Each increment increase in the pressure of thefractionator column will be performed along with an incremental increasein bottom temperature, to avoid a breakthrough of the light ends in thebottom product. Further, the reflux flow will be incrementally increasedto lower the probability that heavier ends will escape from the columnoverhead. In addition, the reflux flow will also be increased due to thehigher boiling rate caused by the higher bottom temperature.

The operating changes described herein were tested to determine theeffects on operations and product purity as described with respect toFIGS. 4-6. The techniques were tested under various operatingconditions, while measuring operating pressure, feed composition, flowrates, and temperature.

FIG. 4 is a plot 400 of pressure 402 versus time 404 showing a change inthe operating characteristics for the columns in the NGL plant 100. Theplot 400 covers a single month of operations in the NGL plant 100,during which the pressure changes took place on day 19. As shown in theplot 400, the fractionator column pressure 406 was initially set atabout 400 psig (about 2860 kPa), while the absorber column pressure 408was initially set to about 365 psig (about 2515 kPa). On day 19, theabsorber column pressure 408 was increased to about 390 psig (about 2690kPa) while the fractionator column pressure 406 was decreased to about350 psig (about 2410 kPa). FIG. 5 discusses the changes made to theabsorber reflux to control the process after the change in pressure, andFIG. 6 discusses the effects on the product specifications

FIG. 5 is a plot 500 of flow rate 502 for reflux to the absorber columnversus time 404 showing the change in operating characteristics for theNGL plant 100. The reflux flow rate was initially increased at the time504 of the pressure change. This allowed more separation for C3+compounds in the rectifying section and also to flash off methane andethane from both the absorber column and the reflux drum. Further, thisallowed operations to control the flow of the overhead stream from bothcolumns.

FIG. 6 is a plot 600 of compositions of products for the NGL plant 100in mole percent 602 versus time 404, showing the change in compositionsduring the process change. The ethane content 604 in the NGL productdropped from 0.5 mole % to 0.1 mole % when the pressure of thefractionator column was reduced 606 by 50 psig. This allowed moreflashing of the ethane to the overhead stream.

A small improvement was seen in the propane in the overhead stream 608from the absorber column as the increase in the reflux allowed betterseparation of the propane from the feed stream by quenching from thecold reflux. The propane in the overhead stream 610 from thefractionator column showed very little change.

Accordingly, the techniques described herein provide reliable and robustoperations for a prolonged period. Further, with the powering down orremoval of the fractionating feed pumps, the techniques are moreefficient in terms of energy composition. Elimination of the pumpsreduces operating expenses for plant installations and capitalexpenditures for new installations. The techniques are applicable forany typical NGL recovery processing plant. It revealed that the proposedline up is reliable to provide robust operations for a prolonged periodand more efficient in term of energy consumption. Furthermore, this setup is applicable for any typical NGL recovery processes.

Other implementations are also within the scope of the following claims.

What is claimed is:
 1. A control system for operating columns in anatural gas liquids (NGL) plant, comprising: a flow sensor to measure aflow rate of a fractionator column feed stream; an absorber columnpressure controller, comprising: an absorber column pressure sensor; andan absorber column pressure control valve; a fractionator columnpressure controller, comprising: a fractionator column pressure sensor;and a fractionator column pressure control valve; and a controller,comprising: a sensor interface to obtain measurements from: the flowsensor; the absorber column pressure sensor; and the fractionator columnpressure sensor; a controller interface to communicate set points to:the absorber column pressure controller; and the fractionator columnpressure controller; a processor configured to execute storedinstructions; and a data store, comprising instructions configured todirect the processor to: read measurements from: the flow sensor; theabsorber column pressure sensor; and the fractionator column pressuresensor; calculate a set point for the absorber column pressurecontroller based, at least in part, on a set point for the flow rate;and adjust the set point for the absorber column pressure controller tomatch the calculated value of the set point.
 2. The control system ofclaim 1, wherein the data store comprises instructions configured todirect the processor to: calculate a pressure differential between anabsorber column and a fractionation column based, at least in part, onthe set point for the flow rate; calculate a new value for the set pointfor the absorber column pressure controller based, at least in part, onthe pressure differential; and adjust the set point based on the newvalue for the set point for the absorber column pressure controller. 3.The control system of claim 2, wherein the data store comprisesinstructions configured to direct the processor to maintain the setpoint for the fractionator column pressure controller, while providingthe new set point to the absorber column pressure controller.
 4. Thecontrol system of claim 1, wherein the data store comprises instructionsconfigured to direct the processor to: calculate a set point for thefractionator column pressure controller based, at least in part, on theset point for the flow rate; and adjust the set point for thefractionator column pressure controller to match the calculated for theset point for the fractionator column pressure controller.